As traditionally viewed in the scientific literature, a crystal is a solid in which the constituent atoms or molecules are arranged in a regular, repeating pattern. The repeating units, often termed "unit cells", are identical parallelopipeds stacked in a space-filling array. The vertices of the unit cells are referred to as the crystal lattice or space lattice. The lattice points are occupied by identical ions, atoms, or molecules. In body-centered crystal structures there is by definition another ion, atom, or molecule at the center of each unit cell which may be different from the ones at the lattice points. Alternatively, in face-centered crystal structures, there is an atom or molecule at the center of each face of the unit cell.
All true solids regardless of chemical composition are crystalline in the pure state--that is, they are structurally a lattice of repeating crystal unit cells. Alternatively, amorphous solids such as glasses, resins, and polymers are actually high-viscosity, supercooled fluids that undergo a very slow plastic flow. Chemically, the ability to obtain any given substance in crystalline form is to a large extent a measure of its purity; and this has provided a now classical means for its identification. Methods of qualitative analysis were developed in the 18th and 19th centuries for identification of a given composition by the ability to crystallize it; and subsequently to identify the substance by its melting point and refractive index. The microscope was first used in analyis by F. V. Raspail [Noureau System de Chimie Organique Fonde Sur Des Methodes d'Observation, Paris, 1833] who used the crystal habit of a solid as a means of identifying chemical compounds. This pioneer work led others to utilize chemical microscopy as a method of identifying crystalline solids. Such methods have long since become standardized procedures [Cheronis and Entrikin, Systematic Semi-Micro Qualitative Organic Analysis, 2nd edition, 1947]; and standard handbooks of physics and chemistry commonly provide page after page of chemical compositions and formulas, each of which has been identified and evaluated by its crystalline melting point and crystalline refractive index as a basis for their individual identity [Handbook Of Chemistry And Physics, 56th edition, CRC publications, 1975-1976]. On this basis also, investigative efforts have been made regarding reactions and methods for preparing many different chemical compositions in crystalline form. Such investigations have included: examination of the various crystal systems and crystalline structures which could exist; means of inducing crystal formation; and methods for obtaining different chemical formula and structure in a crystalline state. Representative of these investigative efforts are the individual texts and reviews in: Industrial Engineering Chemistry, volume 61, pages 65-101, 1969, and volume 62, pages 148-155, 1970; Cheronis and Entrikin, Identification Of Organic Compounds, 1963; Cheronis, Entrikin, and Hodnett, Semi-Micro Qualitative Organic Analysis, 3rd edition, 1965; Koler, L. and Kofler, A., Mikromeihoden Zur Kennzeichnung Organische Stoffe U Stoffgemische, Innsbruck, 1948; Shriner, Fusow, and Curtin, The Systemic Identification Of Organic Compounds. 4th edition, 1956; Behren's-Kley, Organische Mikrochemische Analysis (translated, Richard E. Sterens), Microscope publications Ltd, 1969.
Only recently has there been any major deviation from the traditional means for identifying crystalline substances via their physical properties of melting point; refractive index; or crystalline shape, habit, and appearance under the microscope. These innovations have taken the form of assays for the specific detection of carbonyl-containing compounds by the controlled and selective formation of light scattering crystals. These assays are described within U.S. Pat. Nos. 4,380,587 and 4,727,024 respectively. The procedures described within these patents are able to detect aldehydes and ketones exclusively (the carbonyl-containing compounds) by a series of selective chemical reactions which result in the formation of optically detectable crystals as the basis for determining the presence or absence of a carbonyl-containing compound in a test sample. As advantageous as these innovations are within qualitative and quantitative detection assays, they are limited exclusively to the detection of carbonyl-containing compounds--that is, only the detection of aldehydes and ketones as a chemical class. These methods are unsuitable and ineffective for the detection of any other organic analyte of interest; and are equally inappropriate and inoperative for the detection of inorganic analytes under any circumstances. The growing recognition and value of the methods described within these patents has emphasized the continuing absence of accurate and reliable methods for the selective detection of other organic and inorganic substances via the selective formation and growth of light scattering crystals; and has focused the attention of the ordinary practitioner in this art on the many advantages and benefits provided by such analytical assays were such detection methods able to be created and designed in a reproducible and accurate manner. Insofar as is presently known, however, there has been no innovation or expansion upon the self-limiting assay procedures as described.